Sefer Ha’mitzvos Ha’Katzar (the concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
First Section – the Positive Commandments
15. It is a positive commandment for each Jew to write for himself a Torah scroll
As it says in Devarim (Deuteronomy) 31:19, "Write for yourselves this song." The explanation is as follows: Write the Torah that has within it this song, because one is not allowed to write the Torah section by section. And if the person writes it with his own hands, about him it is written that it is like he has accepted the Torah on Mount Sinai. If it is impossible for him to write it for himself, he needs to buy it or hire someone to write it for him. And even though he has a Torah scroll from his ancestors, there is a commandment that he should write one for himself. If he corrects one letter from a Torah scroll, it is like he has written the entire Torah.
The Rosh (Rabenu Asher of blessed memory) writes that all of this applied in earlier generations when they would write a Torah scroll and learn from it, but in our days, when we write a Torah scroll and place it in the synagogue to publicly read from it, it is a positive commandment for each Jew that has the means to buy Chumashim, Mishnayos, Gemara, and their commentaries, in order that he and his sons can learn from them. The commandment to write a Torah scroll is so that we can learn from it. It is through learning the Gemara and its commentaries that he will know the meaning of the mitzvos and their laws thoroughly. Therefore, these are the books that a man is commanded to have written.
One that has within his power to fulfill both of these interpretations, then fortunate is his lot.
This commandment applies everywhere and at every time, for men but not for women.